feat(CSS): add SASS (libsass) preprocessor option

- Enable developers to choose between LESS and SASS preprocessors.
- Make gulp config reference to source of stylesheets general so that only files in styles/ folder are targeted (it will work with both custom.less and custom.scss).
- Set up a more granular styles folder architecture as per http://docssa.info/ that helps in maintaining them.
- Reference changes in README file.